1. Field of the Invention
This invention relates to a system for electrochemical anodization of silicon substrates and, more specifically, to computer controlled anodization of specially prepared porous silicon substrates.
2. Brief Description of the Prior Art
Anodization of bulk silicon generally takes place in order to create silicon having a density which is about half that of the bulk silicon due to the formation of pores in the anodized region. This pore formation provides an increase in surface area of the anodized silicon and permits the anodized region to be oxidized more rapidly than the bulk silicon. Anodization is often used to form isolated islands of silicon within the wafer bulk. An example of the formation of such islands involves starting with N- bulk silicon, forming an N+ layer thereon followed by an N- layer. The isolated island regions are then masked with a layer of silicon nitride followed by a layer of silicon oxide and the unmasked regions are then etched to a level into the bulk to provide a trench exposing the N+ layer. Anodization now forms pores in the N+ layer and subsequent oxidation causes formation of an oxide layer in the region of the former N+ layer as well as along the sidewalls of the trench.
In the prior art of anodization of silicon substrates, it has been necessary to make direct ohmic contact to the substrate being anodized via some metallizing scheme in order to monitor and control the voltage drop thereacross during anodization. Such monitoring is necessary in order to avoid deleterious effects in anodization of the substrate due to changes in the voltage drop across the wafer and, hence, the anodization current density as well as changes due to anodizable area changes. Constant current density is critical to the uniformity of the anodization process and to the subsequent oxidation process since non-uniformities in current density during the anodization process lead to variations in the sizes of the resulting pores in the porous layer, particularly for features 30 microns wide or less. Variable pore size, in turn, leads to oxidation induced stresses which cause unacceptable defect levels in the isolated islands formed. In addition, changes in electrolyte concentration which occur with depletion of the electrolyte can result in electrochemical etching as opposed to anodization. Such prior art related to anodization and methods thereof are set forth in U.S. Pat. No. 4,628,591 and pending application Ser. No. 806,258, filed Dec. 6, 1985 of Zorinsky et al., now abandoned, and Ser. No. 810,001, filed Dec. 17, 1987 of Keen et al., now abandoned, and all assigned to the assignee of the subject application, as well as in the literature referenced in these applications and patent wherein direct contact with the wafer or substrate in some fashion is described. It has been found by the present invention that the problems inherent in systems making direct ohmic contact to the wafer being anodized are lessened by providing a system wherein such direct contact is not required.